The performance of ultra capacitor and lithium batteries electrolytes at low temperature is a continuing problem since the conductivity of the electrolyte will go to zero if it freezes before a desired low temperature performance is achieved. Various blends of organic carbonates have been used along with the addition of ethers and low molecular weight esters to achieve low temperature (−60° C.) freezing points of the mixed solvents containing lithium hexafluorophosphate for low temperature lithium-ion battery performance. The use of mixtures of ethylmethyl carbonate with dimethyl carbonate and small quantities of ethylene carbonate have allowed performance down to −20° C. and even −30° C. in some cases for lithium battery electrolytes. The use of tetrahydrofuran (THF) and methyl formate and methyl acetate and dimethyl ethylene glycol (glyme) or dimethoxy ethane (DME) has allowed some battery electrolytes to achieve −40° C. or even −50° C. performance. The problem is that the performance of these electrolytes at high temperature such as >70° C. causes high vapor pressures in the batteries with these volatile low boiling solvents. In the case of ultra capacitors based on organic electrolytes the situation is similar except most current ultra capacitor electrolytes are based on the use of acetonitrile (low boiling with by 82° C.) containing tetraethylammonium tetrafluoroborate. These ultra capacitor electrolytes have an upper operating voltage limitation of 2.7 V. The use of capacitor electrolyte solvents based on the organic carbonates and containing tetraalkylammonium tetrafluoroborates has also been limited because of solubility limitations when using propylene carbonate, or by low temperature performance when using mixtures containing ethylene carbonate.
Propylene carbonate has been used in mixtures with other organic carbonates for capacitor electrolytes but this solvent also limits cell voltage to about 3 V and the solubility of the tetrafluoroborate salt decreases rapidly on cooling and results in low conductivity of the electrolyte at temperatures below −20° C. The use of ethylene carbonate (mp 35° C.) with cyclic organic carbonate mixtures containing organic quaternary tetrafluoroborate salts for capacitor use gives higher operating cell voltage in ultra capacitors, but these electrolytes freeze before −20° C. is reached. Low temperature cycling performance (non-freezing) is desired (required for the use of ultra capacitor performance in vehicle performance down to −30° C.). In aircraft, the temperature desired is down to −40° C. At the same time these applications desire high temperature performance (>70° C.) with low vapor pressure. This means that volatile solvents which are used for low temperature performance cause problems at the high end of the desired performance range.
Activated carbon is the preferred material for use in preparing electrodes for carbon electrode capacitors. This activated carbon is prepared from a number of different sources such as coconut shells, wood, sugar, cellulosics and phenolic resins. After converting these materials to carbon under steam controlled conditions, the carbons are “activated” in a second step using steam or catalyzed with KOH, NaOH and/or carbon dioxide and KOH to increase the surface area to very high surface areas such as 1000 to 2400 m2/g. These activated carbons usually contain about 2% oxygen after they have been thoroughly dried and traces of inorganic salts. This oxygen is probably present as quinones, hydroquinones, esters, phenols, carboxylic acids, furans and possibly ketones etc. with some nitrogen compounds present—all of which under high voltage conditions greater than 3 V. will undergo electrochemical oxidation/reduction as the voltage is increased past 3.3 V. At lower voltages, these functional groups actually improve the energy storage capacity of the carbon and are desirable at voltages below 3.2 V.
The basic components of electrical capacitors include conductive electrodes connected to an electric power supply and a dielectric material separating the electrodes. Electrolytic capacitors and electrochemical double layer capacitors also have an electrolyte. In an electrolyte capacitor, the electrodes are provided by an oxide or carbon layer formed on metal foil and are separated by a porous non-conducting membrane such as paper, porous polymer, etc. The liquid electrolyte provides electrical contact to the opposite electrode through the separator. The inherently high resistance of electrolytic capacitors is generally mitigated by rolling a large sheet of the electrode material into a roll to give high surface area. In an electrochemical double layer capacitor, the dielectric is provided by the electrolyte. In this type of capacitor the resistance of the electrolyte is a significant factor in the total device resistance. In capacitors that use electrolytes, the temperature has a major influence on the electrolyte in the performance of the capacitor since the conductivity of the electrolyte decreases with temperature.
Electrochemical double layer capacitors including super capacitors, typically comprise electrodes, electrical contacts to a power supply, separators for electrodes and/or cells, and electrolyte and environmental seals. As mentioned above, a key component of electrolytic and electrochemical double layer capacitors is the electrolyte, which typically comprises a combination of a conductive salt and a solvent. Desirable electrolytes are typically liquid with low viscosity, low density, and high conductivity over a range of ambient temperature conditions. They should also be commercially inexpensive, chemically and electrochemically stable, and compatible with carbon. Aqueous electrolyte systems have been used extensively and provide voltage restricted below 1.8 V. For example, ultra capacitors in Japan are not permitted to use acetonitrile for the electrolyte. A need exists for improved electrolyte systems that provide optimum capacitance for capacitors to achieve high power density, a wide temperature range, and a long lifetime without memory effects.
The key requirements for the electrolyte in both non-aqueous batteries and capacitors are high voltage stability, low temperature performance and electrochemical stability. U.S. Pat. No. 6,743,947 to Xu, et al. discloses an electrolyte system comprising a mixture of ethylene carbonate and dimethyl carbonate at a concentration of the electrolyte salt at 0.5−2.5 M which has poor conductivity at low temperatures.
U.S. Pat. No. 7,924,549 to Smith, et al discloses conditioned carbon electrodes for capacitors and lithium batteries having conditioned carbon electrodes that have also been heat treated and used in an electrolyte comprising a quaternary ammonium tetrafluoroborate salt in an aprotic solvent.
U.S. Patent Application Publication No. 20070002522 discloses capacitors having electrodes with alkali-activated carbon electrodes in electrolytic solutions comprising a quaternary ammonium salt and a solvent containing a carboxylic ester.
Electrochemical double layer capacitors capable of high energy density, known as “super-capacitors”, have been assembled from a variety of materials. In general, it is desirable to construct super-capacitors with light weight materials and electrolytes that are stable and non-reactive, as described in U.S. Pat. No. 5,260,855 issued to Kaschmitter et al., the teachings of which are hereby incorporated by reference. This type of super-capacitor incorporates electrodes based on carbon that may be prepared from organic gels.
U.S. Pat. No. 6,902,683 to Smith et al., which is herein incorporated by reference, relates to electrolytes of a complex salt formed by mixing of a tetraalkyl ammonium salt of hydrogen fluoride with an imidazolium compound in a nitrile solvent which operate at temperatures between −60 and 150 degrees C.
The article of Ue in J. Electrochem. Soc. Vol 141, No. 11, November 1994 entitled “Electrochemical Properties of Organic Liquid Electrolytes Based on Quaternary Onium Salts for Electrical Double-Layer Capacitors” which is herein incorporated by reference, discloses high permittivity solvents and onium salts for double-layer capacitors. Specifically studied were quaternary onium tetrafluoroborate salts which showed greater solubility in the solvents with good stability and conductivity.
U.S. Pat. No. 6,980,415 to Higono et al, which is herein incorporated by reference, discloses an electrolyte for capacitors comprising dimethyl carbonate and a spiro tetrafluoroborate salt. The tetrafluoroborate salt can be used in the present invention with the present electrolyte solutions.